The calls for violence came over the radio in South Sudan: hate messages, urging people from one ethnic group to commit atrocities against other groups. The result was horrific. Rebels slaughtered hundreds of civilians who had sought refuge in a mosque, church and hospital. The killings took place last week and these gruesome details are just starting to emerge. Last night, the White House called the massacre an abomination.

Toby Lanzer saw the results firsthand. He's the top U.N. humanitarian officer in South Sudan, and he went to the city of Bentiu to document what he describes as heart-wrenching atrocities.

TOBY LANZER: I arrived in Bentiu four days after the violence had subsided and I drove into town along a dirt road. There were bodies lying in the streets. We then drove on churches and public buildings, including a mosque. The mosque, we had understood, was the scene of really a terrible crime where perhaps up to 200 people had been killed. And we went inside the mosque and saw the remains of many, many people. We then proceeded to the hospital, which was also the scene of killing. So I think in the space of perhaps two hours, my team and I saw well over a hundred, probably up to 150 bodies. So very troubling scenes, indeed.

BLOCK: I'm sure. And the bodies that you were seeing were men, women, children, all ages?

LANZER: They were, indeed. And I think what was particularly striking was that almost of them were in civilian clothing. We understand from eyewitness accounts and our own team that was there that there had been messages broadcast over a local FM radio station encouraging people to engage in violence and to target it very deliberately, not only at the level of rape but also to kill people based on their identity.

We are providing assistance to the civilians who were left in town. Most of them, if not all of them, have either fled to the bush or gone up to the United Nations Peacekeeping base which is on the northern edge of the town. A couple of weeks ago, there were still about 4,000 people who were seeking protection inside our base. But that number has now swelled to some 25,000 people.

BLOCK: You're saying that these people are seeking refuge at the U.N. compound. But my understanding is that just immediately after this massacre in Bentiu, people who were seeking refuge at another U.N. compound were, in fact, the victims of another massacre, a revenge attack.

LANZER: Yes, Melissa, indeed. In the city of Bor, just south of Bentiu, a couple of days after events had evolved in Bentiu, we did have a group of youth who were approaching our base in Bor allegedly with a petition that they wanted to hand the United Nations. Upon reaching our gate, they reached the perimeter, opened fire. And while the United Nations was able to repel the attack, it could become particularly difficult for us to do that because the protesters who were armed were dressed almost exclusively in civilian clothing.

So once they had reached our perimeter, it became very difficult to distinguish between attacker and the civilians who were seeking protection inside our base. And, yes, it's terrible that 46 misplaced persons who were seeking protection from the U.N. lost their lives, but it's perhaps of some comfort that 4,950 did not.

BLOCK: It does appear, Mr. Lanzer, that what started out a few months ago as a political struggle between the South Sudanese president who is from the Dinka ethnic group, and then the former vice president, who is from the Nur ethnic group, that that's now devolved into an all-out ethnic struggle in South Sudan. And I wonder where you see this ending.

LANZER: I think that at the moment, it's imperative that the international community stand with the people of South Sudan, first and foremost. Second, call on the leaders, whoever they may be, and in particular the parties to this conflict to reconcile at the table and not via the barrel of the gun to show enlightened leadership right now because what people really need is to stay alive. They need breathing space in the months of April and May. This is precisely the moment when the people of South Sudan should be out planting their fields, cultivating, benefiting from the beginning of the rainy season so that by the end of the year they have a harvest and they can actually fend for themselves.

BLOCK: Toby Lanzer is the top U.N. humanitarian official in South Sudan. Mr. Lanzer, thanks for talking with us.